TXB0108: interface between DS16F95QML and FPGA

Wioleta E

Part Number: TXB0108
Other Parts Discussed in Thread: DS16F95QML, SN74AUP1T34, SN74LV1T34, SN74AXC1T45, SN74LVC1G34, SN74AHC1G125

One of our customer connect our board with FPGA chip and TXB0108 on IO lines to the RS485 transceiver DS16F95QML. Are there any potential problems for such connection?

The RS485 is operated in half duplex mode. By default, on power on, 485 is in receive mode. So with the fail safe biasing present (Vab > 0.2) the output of the RS485 IC is expected to be high when no transmitter is driving the differential line. The customer noticed some issues on the IOs.

Bellow the customer's observations:

- the FPGA is powered OFF -> the output of the RS485 IC is going high.

- the FPGA is powered ON with pull up enabled on the IO -> the output of the RS485 IC is going low.

- the FPGA is powered ON with pull up disabled on the IO -> the output of the RS485 IC is going low.

There are no any external pull-up or pull-down resistors on the board. Could you please help us to understand why the output of the RS485 is going low even if pull up is enabled on the FPGA IO?

Best Regards,

Wioleta

over 7 years ago

0 Clemens Ladisch over 7 years ago

Guru 316470 points

The DS16F95QML's drive strength is too low for the TXB0108; the DS16F95QML's V_OH is specified for at most 400 µA, while the TXB0108 must be driven with at least 2 mA.

But in your circuit, you do not actually need automatic direction sensing; use two unidirectional or direction-controlled translators (or an AHC/LVC buffer for downtranslation).

0 ShreyasRao over 7 years ago in reply to Clemens Ladisch

TI__Guru 61910 points

Hi Clemens,

Thank you for your inputs here.

Wioleta,
You need to consider using the direction controlled translation like the SN74AXC1T45 or unidirectional device like the SN74AUP1T34/ SN74LV1T34.

0 ShreyasRao over 7 years ago in reply to ShreyasRao

TI__Guru 61910 points

In case of only down translation, an overvoltage tolerant input device like the sn74LVC1G34 can work as well.

0 Wioleta E over 7 years ago

Prodigy 30 points

Thank you for all answers. Actually at this stage we do not have the ability to change the leveltranslator, we can optionally add some additional elements that will solve this issue.
I agree that there is no need for a bidirectional leveltranslator for connection beetween FPGA and DS16F95QML transceiver, but our project is very specific and we have to ensure bidirectional transmission on this IOs.

0 Clemens Ladisch over 7 years ago in reply to Wioleta E

Guru 316470 points

You could put a buffer (e.g., SN74AHC1G125) between the receiver and the TXB.

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TXB0108: interface between DS16F95QML and FPGA